This invention relates to cigarette filters.
The increasing use of filters in cigarettes, not only for the purpose of removing tars and other undesirable substances from the tobacco smoke but also to save the cost of tobacco which would otherwise be thrown away in the butt-end, has led to the investigation and development of many kinds of filters.
Cigarette filters made for instance of crimped paper or cellulose acetate tow have met with commercial success, though these entail the use of relatively complex machinery for handling the loose starting materials which must be rolled in paper or otherwise bound together into the desired shape of filter before being incorporated in the cigarette. These particular types of filter can also be comparatively heavy. Many other types of filter have been investigated, among which are certain kinds of filters of foamed plastics. One problem here is that there is liable to be a free flow of tobacco smoke through the filter but an inadequate absorption of the substance it is desired to remove.
The best types of cigarette filter need to have an agreeable feel to the lips yet resist damage by highspeed cigarette-making machinery, they need to exert less than a certain degree of hindrance to the passage of tobacco smoke on drawing, and yet they must remove an adequate proportion of the undesirable substances.
These qualities are not easy to obtain. It is possible for instance by foaming such resins as polypropylene to obtain foamed products which sometimes have a potential as a cigarette filter. Nonetheless, not all such foams are suitable; some are quite useless and others very inefficient. For instance, it is evident in any event that an open-cell structure is necessary, because otherwise there will be no effective passage of the tobacco smoke through the filter at all. In some examples of open-cell foam, however, the tobacco smoke passes through the filter almost unhindered, with little or no removal of the undesirable substances present in the smoke. Thus foamed polyethylene and foamed polypropylene filters of various types have been proposed, but it has not previously been suggested that it is possible by forming a micro-structure of the nature described later in this specification to obtain a filter having the characteristics set out below, nor has any process for producing cigarette filters been described which would teach how to obtain such a microstructure.